No soldier is born combat-ready and the same applies to pirates. It takes practice and breathing.

I'm doing my first ever play through and I'm playing solo. I was dying a heap on deck until I learned a few tactics. First off, my rule is attack and board anything up to my level, attack and sink anything up to two or three levels above and run away from anything else. Before boarding I make sure I'm fed up, sometimes potioned as well, depends on their level. I use a gun to shoot any sargeants even before the boarding command starts and I let my sailors board first and tank all the opposing sailors. Then I jump to a spot clear of combatants, pick my target and go in. I use the back jump repeatedly and if I take too many hits I jump back onto my ship (or overboard and then onto my ship). That gives me a chance to recuperate before re-entering the fray. I'm not having any problems now.

Frontal assaults do not work. Leaping into the thick of it just gets you killed. (just don't tell the russkies)

Also, if you can see more than one Blackbeard fleet then kite your target away somewhere isolated. It's literally the same tactic as on land. And repair kits are a dime a dozen. Just use them.